Holmes is aware that the Red-Headed League was to prosperous to be real. Therefore, he concludes that it must have been a ploy. Moreover, Spaulding was willing to work for very little money and spent most of his time in the basement. As there was a bank nearby, Holmes concluded that Spaulding must have been digging a tunnel to rob the bank. Knowing that the robbery was going to happen soon (because of the dissolution of the league), he was able to capture John Clay.
